Impedance measurement

ABSTRACT

Accurately measuring bio-impedance is important for sensing properties of the body. Unfortunately, contact impedances can significantly degrade the accuracy of bio-impedance measurements. To address this issue, circuitry for implementing a four-wire impedance measurement can be configured to make multiple current measurements. The multiple current measurements set up a system of equations to allow the unknown bio-impedance and contact impedances to be derived. The result is an accurate bio-impedance measurement that is not negatively impacted by large contact impedances. Moreover, bad contacts with undesirably large impedances can be identified.

PRIORITY APPLICATIONS

This continuation patent application claims priority to and receivesbenefit from U.S. Non-Provisional application Ser. No. 16/374,021,titled “BIO-IMPEDANCE AND CONTACT IMPEDANCES MEASUREMENT”, filed on Apr.3, 2019. The US Non-Provisional Application claims priority to andreceives benefit from U.S. Provisional Application, Ser. No. 62/678,986,titled “BIO-IMPEDANCE AND CONTACT IMPEDANCES MEASUREMENT”, filed on May31, 2018, and U.S. Provisional Application, Ser. No. 62/679,460, titled“BIO-IMPEDANCE AND CONTACT IMPEDANCES MEASUREMENT”, filed on Jun. 1,2018. The US Non-Provisional Application and the two US ProvisionalApplications are hereby incorporated in their entirety.

TECHNICAL FIELD OF THE DISCLOSURE

The present invention relates to the field of integrated circuits, inparticular to impedance measurements.

BACKGROUND

Impedance measurements of the body, referred herein as bio-impedance,has many applications in healthcare and consumer applications. Impedancemeasurements can be made by electrodes provided in body-worn systems, orwearable devices, such as wrist watches, chest bands, head bands,patches, and so on. Circuitry coupled to the electrodes can derive theunknown impedance of the body on which the electrodes are placed.Impedance measurements can be particularly useful for vital-signsmonitoring, sensing of tissues and fluid level in the body for purposesof detecting signs of pulmonary edema, or assess body composition.Moreover, electrical impedance tomography is an emerging non-invasivetechnique of medical imaging. Due to various challenges, making anaccurate bio-impedance measurement is not trivial.

BRIEF DESCRIPTION OF THE DRAWINGS

To provide a more complete understanding of the present disclosure andfeatures and advantages thereof, reference is made to the followingdescription, taken in conjunction with the accompanying figures, whereinlike reference numerals represent like parts, in which:

FIG. 1 illustrates a system having electrodes and circuitry forperforming one exemplary way of making a four-wire impedance measurementof bio-impedance, according to some embodiments of the disclosure;

FIG. 2 illustrates input capacitances present in circuitry that performsa four-wire impedance measurement of bio-impedance, according to someembodiments of the disclosure;

FIG. 3 illustrates current leakage present in circuitry that performs afour-wire impedance measurement of bio-impedance, according to someembodiments of the disclosure;

FIG. 4 illustrates a calibration measurement, according to someembodiments of the disclosure;

FIGS. 5-9 illustrate five current measurements, according to embodimentsof the disclosure;

FIG. 10 illustrates current leakage present in the measurement seen inFIG. 5, according to some embodiments of the disclosure;

FIGS. 11-15 illustrate five current measurements which avoid currentleakage, according to embodiments of the disclosure; and

FIG. 16 is a flow diagram illustrating a method for measuringimpedances, according to some embodiments of the disclosure.

DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE DISCLOSURE

Overview

Accurately measuring bio-impedance is important for sensing propertiesof the body. Unfortunately, contact impedances can significantly degradethe accuracy of bio-impedance measurements. To address this issue,circuitry for implementing a four-wire impedance measurement can beconfigured to make multiple current measurements. The multiple currentmeasurements set up a system of equations to allow the unknownbio-impedance and contact impedances to be derived. The result is anaccurate bio-impedance measurement that is not negatively impacted bylarge contact impedances. Moreover, bad contacts with undesirably largeimpedances can be identified.

Four-Wire Impedance Measurement

One technique for impedance measurement is a four-terminal sensingscheme, or four-wire impedance measurement scheme. Sometimes it isreferred to as Kelvin sensing. The technique involves using fourelectrodes placed on the body to sense or derive an unknownbio-impedance.

FIG. 1 illustrates a system 100 having electrodes and circuitry forperforming one exemplary way making a four-wire impedance measurement ofbio-impedance, according to some embodiments of the disclosure. In theFIGURE, the unknown bio-impedance is shown as Z_(BODY). The system 100includes electrodes 104, 106, 108, and 110 (or contacts to the body).The electrodes 104, 106, 108, and 110 have respective contact impedancesZ_(E1), Z_(E2), Z_(E3), and Z_(E4). Contact impedances Z_(E1), Z_(E2),Z_(E3), and Z_(E4) can represent skin-electrode impedance of theelectrodes 104, 106, 108, and 110, respectively. Circuitry 150, packagedas an integrated circuit or chip, has pins (or connections) to which theelectrodes are connected. Pin CE0 is electrically coupled to electrode104. Pin AIN2 is electrically coupled to electrode 106. Pin AIN3 iselectrically coupled to electrode 108. Pin AIN1 is electrically coupledto electrode 110.

The system 100 has four branches: a branch that includes electrode 104and pin CE0, a branch that includes electrode 106 and pin AIN2, a branchthat includes electrode 108 and pin AIN3, and a branch that includeselectrode 110 and pin AIN1. Two branches are for sensing a first end ofthe unknown bio-impedance Z_(BODY), and two other branches are forsensing a second end of the unknown bio-impedance Z_(BODY). The branchthat includes electrode 104 is coupled to the first end of unknownbio-impedance Z_(BODY). The branch that includes electrode 106 iscoupled to the first end of unknown bio-impedance Z_(BODY). The branchthat includes electrode 108 is coupled to the second end of unknownbio-impedance Z_(BODY). The branch that includes electrode 110 iscoupled to the second end of unknown bio-impedance Z_(BODY). The fourbranches are connected to respective pins of circuitry 150. Parts of thebranches outside of circuitry 150 can represent cables with patches atthe end of the cables. Parts of the branches outside of circuitry 150can also represent conductors or wires having electrodes at the end ofthe conductors or wires. The conductors and electrodes can be fitted ina wearable device. Optionally, capacitances shown C_(ISO1), C_(ISO2),C_(ISO3), C_(ISO4) can be included between respective pairs ofelectrodes and pins to provide isolation and protection between the bodyof the human user and the circuitry within circuitry 150 (e.g., to blockDC signals).

Circuitry 150 can include a multiplexer (mux) 112. Mux 112 can becontrolled in a manner to connect signal paths of the different pins todifferent parts of circuitry 150. Mux 112, as used herein, represents aconfigurable network controllable to connect different parts ofcircuitry 150 to different pins. For instance, mux 112 can connectdifferent parts of circuitry 150 to different branches connected to thepins (the branches having respective electrodes). Differentconfigurations of mux 112 can form different signal paths or differentimpedance networks (impedance networks being synonymous with signalpaths).

Circuitry 150 can include a signal generator 116 (e.g., sinusoidalsignal generator). Signal generator can generate a signal having a peakvoltage of V_(PEAK). The signal generator generates the signal at anoutput of the signal generator.

Circuitry 150 can include voltage measurement circuitry 118 to measure avoltage across a positive input and a negative input of the voltagemeasurement circuitry 118. In some embodiments, voltage measurementcircuitry 118 can include an instrumentation amplifier (inAmp) 120 witha positive terminal and a negative terminal to sense a voltagedifference between the positive terminal and negative terminal, andoutputs a voltage output representative of that voltage difference.Voltage measurement circuitry 118 can include a Discrete FourierTransform (DFT) block 122 and summation block 124 to generate a voltagemeasurement based on the voltage output from inAmp 120. Components forgenerating a voltage measurement (e.g., a difference in voltage betweentwo inputs) can differ depending on the implementation.

Circuitry 150 can further include current measurement circuitry 126 tomeasure a current at an input of the current measurement circuitry 126.In some embodiments, current measurement circuitry 126 can include atransimpedance amplifier (TIA) 128 to convert a current at an inputterminal of the TIA 128 to a voltage output representative of thecurrent. Current measurement circuitry 126 can include a DFT block 130and summation block 132 to generate a current measurement based on thevoltage output from TIA 128. Components for generating a currentmeasurement (e.g., an amount of current flowing through an input) candiffer depending on the implementation.

To make an impedance measurement, a voltage is generated across theunknown bio-impedance shown as Z_(BODY). The voltage across the unknownbio-impedance Z_(BODY) can be viewed as V_(A)-V_(B). The voltage acrossthe unknown bio-impedance Z_(BODY) can be generated or imposed by signalgenerator 116. Meanwhile, the voltage across the unknown bio-impedanceZ_(BODY) is measured by the voltage measurement circuitry 118, and thecurrent through the unknown bio-impedance Z_(BODY) is also measured, bycurrent measurement circuitry 126. The measured voltage and the measuredcurrent can be used to derive the impedance value of the unknownbio-impedance Z_(BODY). Specifically, the impedance value of the unknownbio-impedance Z_(BODY) is related to the voltage measurement divided bythe current measurement.

In conventional two-wire impedance measurements, measurement issues canarise from impedances of cables (including contact impedances) beingadded to the unknown bio-impedance Z_(BODY), thus corrupting theimpedance measurement. For simplicity, the impedances present are lumpedtogether as a contact impedance in each branch. In theory, a four-wireimpedance measurement can avoid such issues. When the unknownbio-impedance Z_(BODY) is much higher than the impedances of the cables,the measurements can be sufficiently accurate.

However, in practice, a four-wire impedance measurement can have certainother limitations or non-idealities that can significantly impact theaccuracy of the bio-impedance measurement. These limitations can besignificant, e.g., when making impedance measurements at lowfrequencies, high frequencies, certain frequencies, or variousfrequencies. In some situations, one or more of the contact impedancesZ_(E1), Z_(E2), Z_(E3), and Z_(E4) can be greater than the unknownbio-impedance Z_(BODY). For instance, mechanical and/or environmentalreasons (e.g., humidity, movement, hair on skin, etc.) can cause poorcontacts, and can severely increase one or more of the contactimpedances. In some severe cases, the (magnitude of) contact impedancescan be greater than 2 kΩ. In some situations, the optional capacitorsC_(ISO1), C_(ISO2), C_(ISO3), C_(ISO4) can also significantly increaseor affect the impedances of the cables. In some situations, the contactimpedances Z_(E1), Z_(E2), Z_(E3), and Z_(E4) can have an imbalance witheach other (e.g., imbalance can be greater than 1 kΩ). These limitationshave been found to degrade the accuracy of the four-wire impedancemeasurement.

One of the problems causing these limitations that degrade the accuracyof the bio-impedance measurement is that there can be large inputcapacitances at pin AIN2 and pin AIN3 (e.g., around 40 pF). FIG. 2illustrates input capacitances present in circuitry that performs afour-wire impedance measurement of bio-impedance, according to someembodiments of the disclosure. Grounded input capacitance 202 can bepresent at pin AIN2, and grounded input capacitance 204 can also bepresent at pin AIN3. Grounded input capacitance 202, contact impedanceZ_(E2), and capacitance C_(ISO2) can form a filter. This filter can beproblematic because the contact impedance Z_(E2) is unknown, and thusthe effect of the filter is unknown as well. Grounded input capacitance204, contact impedance Z_(E3), and capacitance C_(ISO3) can also formanother filter. This other filter can be problematic because the contactimpedance Z_(E3) is unknown, and thus the effect of this other filter isunknown as well. Ideally, voltage V_(A) should be the same as thevoltage V_(C), and voltage V_(B) should be the same as the voltageV_(D). Due to the grounded input capacitances 202 and 204, at certainfrequencies, voltage V_(A) is not the same as the voltage V_(C), andvoltage V_(B) is not the same as the voltage V_(D). The voltage acrossV_(A) and V_(B) may not be the same as the voltage across V_(C) andV_(D). The negative effect of the grounded input capacitances 202 and204 can be observable at low frequencies and when contact impedances arehigh, e.g., in the range of hundreds or thousands of Ohms. Furthermore,the grounded input capacitances 202 and 204 can attribute to imbalancesin the contact impedances. Imbalances in the contact impedances of thebranches can produce different cut-off frequencies, thereby causingdifferent attenuations in each branch.

Another problem that may degrade the accuracy of the bio-impedancemeasurement is current leakage. FIG. 3 illustrates current leakagepresent in circuitry that performs a four-wire impedance measurement ofbio-impedance, according to some embodiments of the disclosure. Thecurrent leakage arises because the impedance Z_(S-) of the branch havingelectrode 108 can be similar to the impedance Z_(F-) of the branchhaving electrode 110 driving the TIA 128. This results in some of thecurrent I_(BODY) flowing through the unknown bio-impedance Z_(BODY) toflow through the branch having electrode 108, and not all of the currentI_(BODY) would flow through the branch having electrode 110. In otherwords, the current I_(ZS-) through the branch having electrode 108 isideally zero, and the current I_(ZF-) through the branch havingelectrode 110 is ideally equal to current I_(BODY). In reality, thecurrent I_(ZS-) is not zero. As a result, the current I_(ZF-) throughthe branch having electrode 110 is not equal to current I_(BODY), andpart of current I_(BODY) is not measured by the current measurementcircuitry 126. The current measurement is corrupted, and thus theimpedance measurement is also corrupted. This issue can be exacerbatedby high contact impedances in the branches.

An Exemplary Scheme for Deriving Contact Impedances Through MultipleMeasurements and Signal Processing

By configuring mux 112 and making multiple current measurements, it ispossible to derive the (unknown) impedances of the system, including theunknown bio-impedance Z_(BODY), and the contact impedances Z_(E1),Z_(E2), Z_(E3), and Z_(E4), based on a system of equations. The systemof equations are formed through a calibration measurement, and severalother current measurements of different signal paths formed byconfiguring mux 112. Mux 112 can selectively couple the output of thesignal generator 116 and the input of the current measurement circuitry126 to different pins (e.g., RCAL1, RCAL2, CE0, AIN2, AIN3, and AIN1).Accordingly, mux 112 can connect the output of the signal generator 116to the input of the current measurement circuitry 126 through differentsignal paths, or different impedance networks involving at least some ofthe unknown impedances. The different signal paths, individually, caninclude two or more of the unknown impedances of the system: the unknownbio-impedance Z_(BODY), and the contact impedances Z_(E1), Z_(E2),Z_(E3), and Z_(E4). Unique signal paths or unique impedance networks ofat least some of the unknown impedances, and the current measurements ofthe unique signal paths or unique impedance networks, setup a system ofequations for the unknown impedances. The unique signal paths or uniqueimpedance networks, together, include each one of the unknown impedancesat least once. Each unique signal path or unique impedance network wouldinclude at least some of the unknown impedances of the system.Effectively, the signal generator 116 can excite unique signal paths orunique impedance networks formed by mux 112, and the current measurementcircuitry 126 can make measurements of current going through the uniquesignal paths or unique impedance networks.

To determine five unknown impedances (the bio-impedance and the fourcontact impedances), at least five equations are needed. With asufficient number of equations, it is possible to derive the fiveunknown impedances through signal processing (i.e., calculations).Through suitable processing, the current measurements allow thebio-impedance and the contact impedances to be determined. The currentmeasurements can be performed by the current measurement circuitry 126.The signal processing can be performed in the digital domain, e.g., bydigital circuitry 190. Digital circuitry 190 can include specializeddigital hardware to perform the signal processing. Digital circuitry 190can include a microprocessor or microcontroller configured to carry outinstructions that implement the signal processing. The digital circuitry190 can be provided on-chip with circuitry 150 or off-chip (as shown).Digital circuitry 190 can be implemented to control mux 112 to formunique signal paths or unique impedance networks from the signalgenerator 116 to the current measurement circuitry 126.Computer-readable storage 192 can store the measurements.Computer-readable storage 192 can store the instructions that implementthe signal processing. The computer-readable storage 192 can be providedon-chip with circuitry 150 or off-chip (as shown).

FIG. 4 illustrates a calibration measurement, according to someembodiments of the disclosure. The calibration measurement is performedto determine a peak voltage from the signal generator 116, if it is notalready measured or if it is not already known. The system of equations(shown as equations 2-6 below) being formed by the current measurementsof unique signal paths going through at least some of the unknownimpedances use the peak voltage measured in the calibration measurementas a numerical constant. The unknown impedances would be derived basedfurther on the peak voltage measured in the calibration measurement.Determining the peak voltage from the signal generator 116 can beperformed in various ways. An output from the signal generator 116 canbe applied to a resistor with a known resistance value, and the currentmeasurement circuitry 126 can measure a current through the resistor.The calibration measurement is represented by: V_(CAL)=I_(CAL)·R_(CAL)(reproduced as equation 1 below). R_(CAL) is a resistor with a knownstable resistance value. I_(CAL) is measured by the current measurementcircuitry 126. Accordingly, V_(CAL), which is the voltage from signalgenerator 116 can be derived.

The resistor with a known resistance value can be provided on-chip withcircuitry 150 or off-chip (as shown). The calibration measurement isoptional if the peak voltage from the signal generator is known. Thecalibration measurement may only need to be performed once, and does notneed to be performed every time impedance measurements are being made.

In the example shown, for the calibration measurement, an (off-chip)resistor R_(CAL) having a known, stable resistance value is coupledacross pins RCAL1 and RCAL2. The mux 112 is configured to couple thesignal path from pin RCAL1 to the signal generator 116 and to couple thesignal path from pin RCAL2 to the current measurement circuitry 126. Themux 112 forms a signal path from the output of signal generator 116 toinput of current measurement circuitry 126, and the signal path includesresistor R_(CAL). The mux 112 connects the output of signal generator116 to input of current measurement circuitry 126 through the resistorR_(CAL). The measured current performed by current measurement circuitryis I_(CAL). With the known resistance value of the resistor R_(CAL), itis possible to derive the voltage V_(CAL)=I_(CAL)·R_(CAL) across theresistor R_(CAL). The measured current I_(CAL) and the known resistancevalue of resistor R_(CAL) form equation 1, seen below. The voltageV_(CAL) represents the (calibrated) peak voltage from signal generator116. The measurement of the voltage V_(CAL) across R_(CAL) is determinedby measuring a current through R_(CAL), i.e., through the signal paththat includes R_(CAL), by current measurement circuitry 126.

FIGS. 5-9 illustrate five current measurements, according to embodimentsof the disclosure. The five current measurements setup a system of fiveequations, and the five unknown impedances (the bio-impedance and thefour contact impedances) can be derived from solving the system of fiveequations. Note that, in the individual branches, impedances in a cableconnected to a pin and a contact impedance are lumped together andrepresented as a contact impedance (e.g., Z_(E1), Z_(E2), Z_(E3), andZ_(E4)), for simplicity. The contact impedances thus representindividual branch impedances.

In FIG. 5, the mux 112 is configured to couple the signal path from pinCE0 to the output of the signal generator 116 and to couple the signalpath from pin AIN1 to the input of the current measurement circuitry126. The measured current obtained by current measurement circuitry 126is I₁. The measured current I₁, measured current I_(CAL), and the knownresistance value of R_(CAL) form equation 2, seen below. Mux 112 hasformed a signal path from the signal generator 116 to the currentmeasurement circuitry 126. The signal path includes unknown contactimpedance Z_(E1), unknown bio-impedance Z_(BODY), and unknown contactimpedance Z_(E4) (in series). The signal path includes a branch withelectrode 104 and pin CE0, and a branch with electrode 110 and pin AIN1.Equation 2 encapsulates the relationship between the three unknownimpedances Z_(E1), Z_(BODY), and Z_(E4) in the signal path and themeasured current I₁, measured current I_(CAL), and the known resistancevalue of R_(CAL). Note that the product of the measured current I_(CAL)and the known resistance value of R_(CAL) is equivalent to the voltageV_(CAL) obtained from the calibration measurement.

In FIG. 6, the mux 112 is configured to couple the signal path from pinCE0 to the output of the signal generator 116 and to couple the signalpath from pin AIN2 to the input of the current measurement circuitry126. The measured current obtained by current measurement circuitry 126is I₂. The measured current I₂, measured current I_(CAL), and the knownresistance value of R_(CAL) form equation 3, seen below. Mux 112 hasformed a signal path from the signal generator 116 to the currentmeasurement circuitry 126. The signal path includes unknown contactimpedance Z_(E1) and unknown contact impedance Z_(E2) (in series). Thesignal path includes a branch with electrode 104 and pin CE0, and abranch with electrode 106 and pin AIN2. Equation 3 encapsulates therelationship between the two unknown impedances Z_(E1) and Z_(E2) in thesignal path and the measured current I₂, measured current I_(CAL), andthe known resistance value of R_(CAL).

In FIG. 7, the mux 112 is configured to couple the signal path from pinCE0 to the output of the signal generator 116 and to couple the signalpath from pin AIN3 to the input of the current measurement circuitry126. The measured current obtained by current measurement circuitry 126is I₃. The measured current I₃, measured current I_(CAL), and the knownresistance value of R_(CAL) form equation 4, seen below. Mux 112 hasformed a signal path from the signal generator 116 to the currentmeasurement circuitry 126. The signal path includes unknown contactimpedance Z_(E1), unknown bio-impedance Z_(BODY), and unknown contactimpedance Z_(E3) (in series). The signal path includes a branch withelectrode 104 and pin CE0, and a branch with electrode 108 and pin AIN3.Equation 4 encapsulates the relationship between the three unknownimpedances Z_(E1), Z_(BODY), and Z_(E3) in the signal path and themeasured current I₃, measured current Ica, and the known resistancevalue of R_(CAL).

In FIG. 8, the mux 112 is configured to couple the signal path from pinAIN2 to the output of the signal generator 116 and to couple the signalpath from pin AIN1 to the input of the current measurement circuitry126. The measured current obtained by current measurement circuitry 126is I₄. The measured current I₄, measured current Ica, and the knownresistance value of R_(CAL) form equation 5, seen below. Mux 112 hasformed a signal path from the signal generator 116 to the currentmeasurement circuitry 126. The signal path includes unknown contactimpedance Z_(E2), unknown bio-impedance Z_(BODY), and unknown contactimpedance Z_(E4) (in series). The signal path includes a branch withelectrode 106 and pin AIN2, and a branch with electrode 110 and pinAIN1. Equation 5 encapsulates the relationship between the three unknownimpedances Z_(E2), Z_(BODY), and Z_(E4) in the signal path and themeasured current I₄, measured current Ica, and the known resistancevalue of R_(CAL).

In FIG. 9, the mux 112 is configured to couple the signal path from pinAIN3 to the output of the signal generator 116 and to couple the signalpath from pin AIN1 to the input of the current measurement circuitry126. The measured current obtained by current measurement circuitry 126is I₅. The measured current I₅, measured current I_(CAL), and the knownresistance value of R_(CAL) form equation 6, seen below. Mux 112 hasformed a signal path from the signal generator 116 to the currentmeasurement circuitry 126. The signal path includes unknown contactimpedance Z_(E3) and unknown contact impedance Z_(E4) (in series). Thesignal path includes a branch with electrode 108 and pin AIN3, and abranch with electrode 110 and pin AIN1. Equation 6 encapsulates therelationship between the two unknown impedances, Z_(E3) and Z_(E4), inthe signal path and the measured current I₅, measured current I_(CAL),and the known resistance value of R_(CAL).

Calibration V_(CAL) = I_(CAL) · R_(CAL) (eq. 1) Measurement CurrentMeasurement (FIG. 5)${Z_{E\; 1} + 0 + Z_{BODY} + 0 + Z_{E\; 4}} = \frac{I_{CAL} \cdot R_{CAL}}{I_{1}}$(eq. 2) Current Measurement (FIG. 6)${Z_{E\; 1} + Z_{E\; 2} + 0 + 0 + 0} = \frac{I_{CAL} \cdot R_{CAL}}{I_{2}}$(eq. 3) Current Measurement (FIG. 7)${Z_{E\; 1} + 0 + Z_{BODY} + Z_{E\; 3} + 0} = \frac{I_{CAL} \cdot R_{CAL}}{I_{3}}$(eq. 4) Current Measurement (FIG. 8)${0 + Z_{E\; 2} + Z_{BODY} + 0 + Z_{E\; 4}} = \frac{I_{CAL} \cdot R_{CAL}}{I_{4}}$(eq. 5) Current Measurement (FIG. 9)${0 + 0 + 0 + Z_{E\; 3} + Z_{E\; 4}} = \frac{I_{CAL} \cdot R_{CAL}}{I_{5}}$(eq. 6)

With five equations (equations 2-6) and five unknown impedancesZ_(BODY), Z_(E1), Z_(E2), Z_(E3), and Z_(E4), the values for the fiveunknown impedances Z_(BODY), Z_(E1), Z_(E2), Z_(E3), and Z_(E4) can bederived and determined. As illustrated by FIGS. 5-9, each unique signalpath includes two branch impedances. Moreover, as seen in FIGS. 5, 7,and 8, some of the unique signal paths can each include thebio-impedance and two branch impedances. Each unique signal pathincludes at least some of the unknown impedances, and together, theunique signal paths include each unknown impedance at least once.

The five equations (equations 2-6) can be rewritten to equations 7-11that gives the unknown impedances Z_(BODY), Z_(E1), Z_(E2), Z_(E3), andZ_(E4) in terms of one or more ones of the current measurements (one ormore of I₁, I₂, I₃, I₄, and I₅), the measured current I_(CAL), and theknown resistance value of R_(CAL). Digital circuitry 190, such as amicrocontroller or microprocessor, can be implemented to compute theunknown impedances based on the measurements seen in FIGS. 4-9 andequations 7-11. Computer-readable storage 192 can store themeasurements, and instructions for processing the measurements to derivethe impedances.

$Z_{E1} = {\frac{I_{CAL} \cdot R_{CAL}}{2} \cdot \left( {\frac{1}{I_{1}} + \frac{1}{I_{2}} - \frac{1}{I_{4}}} \right)}$(eq. 7)$Z_{E2} = {\frac{I_{CAL} \cdot R_{CAL}}{2} \cdot \left( {\frac{1}{I_{2}} + \frac{1}{I_{4}} - \frac{1}{I_{1}}} \right)}$(eq. 8)$Z_{E3} = {\frac{I_{CAL} \cdot R_{CAL}}{2} \cdot \left( {\frac{1}{I_{3}} + \frac{1}{I_{5}} - \frac{1}{I_{1}}} \right)}$(eq. 9)$Z_{E4} = {\frac{I_{CAL} \cdot R_{CAL}}{2} \cdot \left( {\frac{1}{I_{1}} + \frac{1}{I_{5}} - \frac{1}{I_{3}}} \right)}$(eq. 10)$Z_{BODY} = {\frac{I_{CAL} \cdot R_{CAL}}{2} \cdot \left( {\frac{1}{I_{3}} + \frac{1}{I_{4}} - \frac{1}{I_{2}} - \frac{1}{I_{5}}} \right)}$(eq. 11)

The measurements seen in FIGS. 4-9 can be performed in any order. Insome cases, more than five measurements can be made to generate morethan five equations.

The scheme illustrated by FIGS. 4-9 can have several advantages. Notethat a voltage measurement across the unknown bio-impedance Z_(BODY) isno longer needed (which is normally required in the four-wire impedancemeasurement illustrated by FIG. 1). As a result, an expensive inAmp 120is no longer required in circuitry 150. Furthermore, the error due tothe grounded capacitances at pins AIN2 and AIN3 (acting as a low passfilter), which causes the voltages of V_(A) not being the same as V_(C)and the voltages of V_(B) not being the same as V_(D), is no longerrelevant since a voltage measurement is not being made. Moreover, thescheme can effectively derive five impedances Z_(BODY), Z_(E1), Z_(E2),Z_(E3), and Z_(E4).

Another Exemplary Scheme for Deriving Contact Impedances ThroughMultiple Measurements and Signal Processing

In the previous scheme illustrated by the measurements seen in FIGS.4-9, there is one limitation: current leakage. FIG. 10 illustratescurrent leakage present in the measurement seen in FIG. 5, according tosome embodiments of the disclosure. When making a current measurement,such as current I₁ (as illustrated by FIG. 5), the branches which arenot connected to the signal generator 116 or the current measurementcircuitry 126 ideally has infinite impedance. With infinite impedance,the branches which are not connected to the signal generator 116 or thecurrent measurement circuitry 126 would have zero current. In otherwords, I_(ZE2) (current through branch having electrode 106 and pinAin2) and I_(ZE3) (current through branch having electrode 108 and pinAIN3) are ideally zero. As a result, I_(ZE1) would be equal to I_(BODY)(current through the unknown bio-impedance), and would also be equal toIIIA (current through branch). This would mean that no current isleaking through the branches having electrode 106 and electrode 108, andthe current measurement circuitry 126 is measuring the current throughthe unknown bio-impedance Z_(BODY) accurately (I_(TIA)=I_(BODY)). Inreality, the branches which are not connected to the signal generator116 or the current measurement circuitry 126 do not have infiniteimpedances, and can have grounded capacitances 1002 and 1004 (e.g., inthe pF or μF range). The grounded capacitances 1002 and 1004 representcircuitry (e.g., circuitry in mux 112) capable of sinking current in thebranches. As a result, a part of the current I_(ZE1) may flow throughthe branches which are not connected to the signal generator 116 or thecurrent measurement circuitry 126. This means that I_(ZE2) and I_(ZE3)is not zero, and I_(ZE1) may not be equal to I_(BODY), and may not beequal to IIIA. As a result, current is leaking through the brancheshaving electrode 106 and electrode 108, and the current measurementcircuitry 126 is measuring the current through the unknown bio-impedanceZ_(BODY) inaccurately (I_(TIA)≠I_(BODY)).

To address this limitation, the current measurements setting up a systemof equations having the unknown impedances can be modified.Specifically, the configuration of the mux 112 is adapted for eachmeasurement, and a different system of equations is used for derivingthe unknown impedances. Instead of leaving some of the signal pathsfloating, all signal paths are connected either to the signal generator116 or the current measurement circuitry 126. The unique signal paths orunique impedance networks, instead of each including just a subset ofthe unknown impedances or just two of four branches, the unique signalpaths or unique impedance networks would include all of thebio-impedance and the branch impedances, and all four branches. As aresult, leaked current can be captured by the system of equations.

For four current measurements, one of the signal paths is connected tothe signal generator 116, and the other three of the signal paths areconnected to the current measurement circuitry 126. One of the fourbranches is connected to the output of signal generator 116, and threeother ones of the four branches are connected to the input of currentmeasurement circuitry 126. For another current measurement, two signalpaths are connected to the signal generator 116, and the other two ofthe signal paths are connected to the current measurement circuitry 126.Two of the four branches are connected to the output of signal generator116, and two other ones of the four branches are connected to the inputof current measurement circuitry 126. Accordingly, no floating brancheswill cause current leakage or sink current. The five currentmeasurements form a different system of equations, since the overallsignal path formed by the mux 112 from the signal generator 116 to thecurrent measurement circuitry 126 now involves parallel impedances(i.e., parallel unknown impedances). However, the system of equationshaving five equations can still enable the five unknown impedances to bedetermined.

By configuring mux 112 and making multiple current measurements, it ispossible to derive the unknown impedances of the system, including theunknown bio-impedance Z_(BODY), and the contact impedances Z_(E1),Z_(E2), Z_(E3), and Z_(E4), based on a system of equations. The systemof equations are formed through a calibration measurement, and severalcurrent measurements of different, unique signal paths formed byconfiguring mux 112. Mux 112 can selectively couple the output of thesignal generator 116 and the input of the current measurement circuitry126 to different pins (e.g., RCAL1, RCAL2, CE0, AIN2, AIN3, and AIN1).Accordingly, mux 112 can connect the output of the signal generator 116to the input of the current measurement circuitry 126 through differentsignal paths, or different impedance networks involving all of theunknown impedances. The different, unique signal paths, form uniqueimpedance networks, where each unique impedance network combines all ofthe unknown impedances of the system: the unknown bio-impedanceZ_(BODY), and the contact impedances Z_(E1), Z_(E2), Z_(E3), and Z_(E4),with a unique topology. Unique signal paths or unique impedance networkseach involving all of the unknown impedances, and the currentmeasurements of the unique signal paths or unique impedance networks,setup a system of equations for the unknown impedances. Effectively, thesignal generator 116 can excite unique signal paths or unique impedancenetworks formed by mux 112, and the current measurement circuitry 126can make measurements of current going through the unique signal pathsor unique impedance networks.

To determine five unknown impedances (the bio-impedance and the fourcontact impedances), at least five equations are needed. With asufficient number of equations, it is possible to derive the fiveunknown impedances through signal processing (i.e., calculations).Through suitable processing, the current measurements allow thebio-impedance and the contact impedances to be determined. The currentmeasurements can be performed by the current measurement circuitry 126.The signal processing can be performed in the digital domain, e.g., bydigital circuitry 190. Digital circuitry 190 can include specializeddigital hardware to perform the signal processing. Digital circuitry 190can include a microprocessor or microcontroller configured to carry outinstructions that implement the signal processing. The digital circuitry190 can be provided on-chip with circuitry 150 or off-chip (as shown).Digital circuitry 190 can be implemented to control mux 112 to formunique signal paths or unique impedance networks from the signalgenerator 116 to the current measurement circuitry 126.Computer-readable storage 192 can store the measurements.Computer-readable storage 192 can store the instructions that implementthe signal processing. The computer-readable storage 192 can be providedon-chip with circuitry 150 or off-chip (as shown).

In this modified scheme, the calibration measurement can be performedbased on the configuration seen in FIG. 4 and equation 1, which yieldsV_(CAL). FIGS. 11-15 illustrate five current measurements, according toembodiments of the disclosure. The five current measurements setup asystem of five equations, and the five unknown impedances (thebio-impedance and the four contact impedances) can be derived fromsolving the system of five equations. Note that, in the individualbranches, impedances in a cable connected to a pin is lumped togetherand represented as a contact impedance (e.g., Z_(E1), Z_(E2), Z_(E3),and Z_(E4)), for simplicity. The contact impedances thus representindividual branch impedances.

In FIG. 11, the mux 112 is configured to couple the signal path from pinCE0 to the output of the signal generator 116, to couple the signal pathfrom pin AIN2 to the input of current measurement circuitry 126, tocouple the signal path from pin AIN3 to the input of current measurementcircuitry 126, to couple the signal path from pin AIN1 to the input ofcurrent measurement circuitry 126. The measured current done by currentmeasurement circuitry 126 is I₁. The configuration of mux 112 in FIG. 11forms an overall signal path that includes Z_(E1) in series with (Z_(E2)in parallel with (Z_(BODY) in series with (Z_(E3) and Z_(E4) inparallel)). The branch with electrode 104 and pin CE0 is connected tothe output of the signal generator 116. The branch with electrode 106and pin AIN2 is connected to the input of current measurement circuitry126. The branch with electrode 108 and pin AIN3 is connected to theinput of current measurement circuitry 126. The branch with electrode110 and pin AIN1 is connected to the input of current measurementcircuitry 126. The measured current I₁, measured voltage V_(CAL), formequation 12, seen below. Equation 12 encapsulates the relationshipbetween the measured current I₁, measured voltage V_(CAL), and theunknown impedances in the overall signal path from the signal generator116 to current measurement circuitry 126 (formed by the mux 112 in theconfiguration shown in FIG. 11).

In FIG. 12, the mux 112 is configured to couple the signal path from pinAIN2 to the output of signal generator 116, to couple the signal pathfrom pin CE0 to the input of current measurement circuitry 126, tocouple the signal path from pin AIN3 to the input of current measurementcircuitry 126, to couple the signal path from pin AIN1 to the input ofcurrent measurement circuitry 126. The measured current done by currentmeasurement circuitry 126 is I₂. The configuration of mux 112 in FIG. 12forms an overall signal path that includes Z_(E2) in series with (Z_(E1)in parallel with (Z_(BODY) in series with (Z_(E3) and Z_(E4) inparallel))). The branch with electrode 104 and pin CE0 is connected tothe input of the current measurement circuitry 126. The branch withelectrode 106 and pin AIN2 is connected to the output of signalgenerator 116. The branch with electrode 108 and pin AIN3 is connectedto the input of current measurement circuitry 126. The branch withelectrode 110 and pin AIN1 is connected to the input of currentmeasurement circuitry 126. The measured current I₂, measured voltageV_(CAL), form equation 13, seen below. Equation 13 encapsulates therelationship between the measured current I₂, measured voltage V_(CAL),and the unknown impedances in the overall signal path from the signalgenerator 116 to current measurement circuitry 126 (formed by the mux112 in the configuration shown in FIG. 12).

In FIG. 13, the mux 112 is configured to couple the signal path from pinAIN3 to the output of signal generator 116, to couple the signal pathfrom pin CE0 to the input of current measurement circuitry 126, tocouple the signal path from pin AIN2 to the input of current measurementcircuitry 126, to couple the signal path from pin AIN1 to the input ofcurrent measurement circuitry 126. The measured current done by currentmeasurement circuitry 126 is I₃. The configuration of mux 112 in FIG. 13forms an overall signal path that includes Z_(E3) in series with (Z_(E4)in parallel with (Z_(BODY) in series with (Z_(E1) and Z_(E2) inparallel))). The branch with electrode 104 and pin CE0 is connected tothe input of the current measurement circuitry 126. The branch withelectrode 106 and pin AIN2 is connected to input of the currentmeasurement circuitry 126. The branch with electrode 108 and pin AIN3 isconnected to the output of signal generator 116. The branch withelectrode 110 and pin AIN1 is connected to the input of currentmeasurement circuitry 126. The measured current I₃, measured voltageV_(CAL), form equation 14, seen below. Equation 14 encapsulates therelationship between the measured current I₃, measured voltage V_(CAL),and the unknown impedances in the overall signal path from the signalgenerator 116 to current measurement circuitry 126 (formed by the mux112 in the configuration shown in FIG. 13).

In FIG. 14, the mux 112 is configured to couple the signal path from pinAIN1 to the signal generator 116, to couple the signal path from pin CE0to the current measurement circuitry 126, to couple the signal path frompin AIN2 to the current measurement circuitry 126, to couple the signalpath from pin AIN3 to the current measurement circuitry 126. Themeasured current done by current measurement circuitry 126 is I₄. Theconfiguration of mux 112 in FIG. 14 forms an overall signal path thatincludes Z_(E4) in series with (Z_(E3) in parallel with (Z_(BODY) inseries with (Z_(E1) and Z_(E2) in parallel))). The branch with electrode104 and pin CE0 is connected to the input of the current measurementcircuitry 126. The branch with electrode 106 and pin AIN2 is connectedto input of the current measurement circuitry 126. The branch withelectrode 108 and pin AIN3 is connected to the input of currentmeasurement circuitry 126. The branch with electrode 110 and pin AIN1 isconnected to the output of signal generator 116. The measured currentI₄, measured voltage V_(CAL), form equation 15, seen below. Equation 15encapsulates the relationship between the measured current I₄, measuredvoltage V_(CAL), and the unknown impedances in the overall signal pathfrom the signal generator 116 to current measurement circuitry 126(formed by the mux 112 in the configuration shown in FIG. 14).

In FIG. 15, the mux 112 is configured to couple the signal path from pinCE0 to the signal generator 116, to couple the signal path from pin AIN2to the signal generator 116 (as well), to couple the signal path frompin AIN3 to the current measurement circuitry 126, to couple the signalpath from pin AIN1 to the current measurement circuitry 126. Themeasured current done by current measurement circuitry 126 is I₅. Theconfiguration of mux 112 in FIG. 15 forms an overall signal path thatincludes (Z_(E1) and Z_(E2) in parallel) in series with Z_(BODY) and inseries with (Z_(E3) and Z_(E4) in parallel). The branch with electrode104 and pin CE0 is connected to the output of the output of signalgenerator 116. The branch with electrode 106 and pin AIN2 is connectedto the output of the output of signal generator 116. The branch withelectrode 108 and pin AIN3 is connected to the input of currentmeasurement circuitry 126. The branch with electrode 110 and pin AIN1 isconnected to the input of current measurement circuitry 126. Themeasured current Is, measured voltage V_(CAL), form equation 16, seenbelow. Equation 16 encapsulates the relationship between the measuredcurrent Is, measured voltage V_(CAL), and the unknown impedances in theoverall signal path from the signal generator 116 to current measurementcircuitry 126 (formed by the mux 112 in the configuration shown in FIG.15).

An alternative to the signal path illustrated by FIG. 15 is to connectthe branch with electrode 104 and pin CE0 and the branch with electrode106 and pin AIN2 is connected to the input of current measurementcircuitry 126, and to connect the branch with electrode 108 and pin AIN3and the branch with electrode 110 and pin AIN1 to the output of signalgenerator 116.

Current Measurement (FIG. 11)$I_{1} = \frac{V_{CAL}}{Z_{E\; 1} + \left( {Z_{E2}//\left( {Z_{BODY} + \left( {Z_{E\; 3}//Z_{E\; 4}} \right)} \right)} \right)}$(eq. 12) Current Measurement (FIG. 12)$I_{2} = \frac{V_{CAL}}{Z_{E2} + \left( {Z_{E1}//\left( {Z_{BODY} + \left( {Z_{E3}//Z_{E4}} \right)} \right)} \right)}$(eq. 13) Current Measurement (FIG. 13)$I_{3} = \frac{V_{CAL}}{Z_{E3} + \left( {Z_{E4}//\left( {Z_{BODY} + \left( {Z_{E1}//Z_{E2}} \right)} \right)} \right)}$(eq. 14) Current Measurement (FIG. 14)$I_{4} = \frac{V_{CAL}}{Z_{E4} + \left( {Z_{E3}//\left( {Z_{BODY} + \left( {Z_{E1}//Z_{E2}} \right)} \right)} \right)}$(eq. 15) Current Measurement (FIG. 15)$I_{5} = \frac{V_{CAL}}{\left( {Z_{E1}//Z_{E2}} \right) + Z_{BODY} + \left( {Z_{E3}//Z_{E4}} \right)}$(eq. 16)${\text{Notation~~for~~parallel~~impedances:}Z_{1}}//{Z_{2} \equiv \frac{Z_{1}Z_{2}}{Z_{1} + Z_{2}}}$Current Measurement (FIG. 11)$I_{1} = \frac{V_{CAL}}{Z_{E1} + \frac{Z_{E2} \cdot \left( {{Z_{E3}Z_{E4}} + {Z_{BODY}Z_{E3}} + {Z_{BODY}Z_{E4}}} \right)}{{Z_{E2}Z_{E3}} + {Z_{E2}Z_{E4}} + {Z_{E3}Z_{E4}} + {Z_{BODY}Z_{E3}} + {Z_{BODY}Z_{E4}}}}$(eq. 17) Current Measurement (FIG. 12)$I_{2} = \frac{V_{CAL}}{Z_{E2} + \frac{Z_{E1} \cdot \left( {{Z_{E3}Z_{E4}} + {Z_{BODY}Z_{E3}} + {Z_{BODY}Z_{E4}}} \right)}{{Z_{E1}Z_{E3}} + {Z_{E1}Z_{E4}} + {Z_{E3}Z_{E4}} + {Z_{BODY}Z_{E3}} + {Z_{BODY}Z_{E4}}}}$(eq. 18) Current Measurement (FIG. 13)$I_{3} = \frac{V_{CAL}}{Z_{E3} + \frac{Z_{E4} \cdot \left( {{Z_{E1}Z_{E2}} + {Z_{BODY}Z_{E1}} + {Z_{BODY}Z_{E2}}} \right)}{{Z_{E4}Z_{E1}} + {Z_{E4}Z_{E2}} + {Z_{E1}Z_{E2}} + {Z_{BODY}Z_{E1}} + {Z_{BODY}Z_{E2}}}}$(eq. 19) Current Measurement (FIG. 14)$I_{4} = \frac{V_{CAL}}{Z_{E4} + \frac{Z_{E3} \cdot \left( {{Z_{E1}Z_{E2}} + {Z_{BODY}Z_{E1}} + {Z_{BODY}Z_{E2}}} \right)}{{Z_{E3}Z_{E1}} + {Z_{E3}Z_{E2}} + {Z_{E1}Z_{E2}} + {Z_{BODY}Z_{E1}} + {Z_{BODY}Z_{E2}}}}$(eq. 20) Current Measurement (FIG. 15)$I_{5} = \frac{V_{CAL}}{Z_{BODY} + \frac{Z_{E1}Z_{E2}}{Z_{E1} + Z_{E2}} + \frac{Z_{E3}Z_{E4}}{Z_{E3} + Z_{E4}}}$(eq. 21)

Equations 17-21 show equations 12-16 in an expanded form based on thenotation for parallel impedances.

With five equations (equations 12-16) and five unknown impedancesZ_(BODY), Z_(E1), Z_(E2), Z_(E3), and Z_(E4), the values for the fiveunknown impedances Z_(BODY), Z_(E1), Z_(E2), Z_(E3), and Z_(E4) can bederived and determined. As illustrated by FIGS. 11-15, each uniquesignal path includes all of the unknown impedances. Moreover, as seen inFIGS. 5, 7, and 8, some of the unique signal paths can each include thebio-impedance and two branch impedances. Each unique signal pathincludes at least some of the unknown impedances, and together, theunique signal paths include each unknown impedance at least once.

Algebraic manipulations can be applied to equations 17-21 to rewriteequations 12-21 so that the unknown impedances Z_(BODY), Z_(E1), Z_(E2),Z_(E3), and Z_(E4) are defined in terms of the current measurements(e.g., I₁, I₂, I₃, I₄, and I₅), the measured current I_(CAL), and theknown resistance value of R_(CAL). The following pseudocode can beimplemented in digital circuitry 190, such as a microcontroller ormicroprocessor, to determine and compute the unknown impedances based onthe measurements seen in FIGS. 4, and 11-15.

vcal = RCAL* ICAL; // calibration measurement illustrated by FIG. 4 a1 =vcal / I1; // current measurement illustrated by FIG. 11 a2 = vcal / I2;// current measurement illustrated by FIG. 12 a3 = vcal / I3; // currentmeasurement illustrated by FIG. 13 a4 = vcal / I4; // currentmeasurement illustrated by FIG. 14 a5 = vcal / I5; // currentmeasurement illustrated by FIG. 15 E1 = −2*a1*a2*a5*(a1*a2 + a1*a5 −a2*a5)/(a1*a1*a2*a2 − 2*a1*a1*a2*a5 + a1*a1*a5*a5 − 2*a1*a2*a2*a5 −2*a1*a2*a5*a5 + a2*a2*a5*a5); // derives Z_(E1) E2 = −2*a1*a2*a5*(a1*a2− a1*a5 + a2*a5)/(a1*a1*a2*a2 − 2*a1*a1*a2*a5 + a1*a1*a5*a5 −2*a1*a2*a2*a5 − 2*a1*a2*a5*a5 + a2*a2*a5*a5); // derives Z_(E2) E3 =−2*a3*a4*a5*(a3*a4 + a3*a5 − a4*a5)/(a3*a3*a4*a4 − 2*a3*a3*a4*a5 +a3*a3*a5*a5 − 2*a3*a4*a4*a5 − 2*a3*a4*a5*a5 + a4*a4*a5*a5); // derivesZ_(E3) E4 = −2*a3*a4*a5*(a3*a4 − a3*a5 + a4*a5)/(a3*a3*a4*a4 −2*a3*a3*a4*a5 + a3*a3*a5*a5 − 2*a3*a4*a4*a5 − 2*a3*a4*a5*a5 +a4*a4*a5*a5); // derives Z_(E4) ZB = (−E1*E2*E3 − E1*E2*E4 − E1*E3*E4 +E1*E3*a5 + E1*E4*a5 − E2*E3*E4 + E2*E3*a5 + E2*E4*a5)/(E1*E3 + E1*E4 +E2*E3 + E2*E4); // derives Z_(BODY)

The measurements seen in FIGS. 4, and 11-15 can be performed in anyorder. In some cases, more than five measurements can be made togenerate more than five equations.

The scheme illustrated by FIGS. 4, and 11-15 can have several advantages(similar to the scheme seen in FIGS. 4-9). Note that a voltagemeasurement across the unknown bio-impedance Z_(BODY) is no longerneeded (which is normally required in the four-wire impedancemeasurement illustrated by FIG. 1). As a result, an expensive inAmp 120is no longer required in circuitry 150. Furthermore, the error due tothe grounded capacitances at pins AIN2 and AIN3 (acting as a low passfilter), which causes to the voltages of V_(A) not being the same asV_(C) and the voltages of V_(B) not being the same as V_(D), is nolonger relevant since a voltage measurement is not being made. Moreover,the scheme can effectively and accurately derive five impedancesZ_(BODY), Z_(E1), Z_(E2), Z_(E3), and Z_(E4). In addition to theseadvantages, this scheme can now ensure accuracy even in the presence ofhigh impedances, and big imbalances between contact impedances.

Additional Technical Advantages

Measuring bio-impedance can be particularly useful for measuring bodyimpedance for detecting fluid level of the lungs or measuring thoracicimpedance. Measuring bio-impedance can also be useful in electricalimpedance tomography to determine a composition of the body (e.g.,imaging of tissues and bones) in a non-invasive manner by makingbio-impedance measurements at different frequencies. Measuringbio-impedance can be useful in measuring respiration activity, whererespiration activity can be obtained by observing variation in thoraximpedance. Measuring bio-impedance and the contact impedances means thatrespiration activity can be obtained even in the presence of motion,since variations in contact impedances can be taken into account. Userssuch as athletes and patients can greatly benefit from suchapplications.

Knowing the contact impedances Z_(E1), Z_(E2), Z_(E3), and Z_(E4) inaddition to the unknown bio-impedance Z_(BODY) can enable the circuitryto infer whether the contacts (i.e., contacts being formed by theelectrodes contacting the body) are good or not, e.g., as part of adiagnostic process. For instance, high contact impedances can indicatethat patches/electrodes are not properly attached to the body.Accordingly, information about the quality of the contacts can beinferred from derived contact impedances.

For example, the digital circuitry 190 can determine quality of contactscorresponding to the four electrodes based on the impedances of the fourbranches. If a given impedance of a branch is too high, the digitalcircuitry 190 can infer that the contact for the branch is bad andoutput a signal that indicates the presence of a bad contact andoptionally an identifier that identifies which contact is bad. Thedigital circuitry 190 can compare the impedances of the four branchesagainst predetermined threshold(s) to determine whether a givenimpedance is too high.

User feedback can be provided based on the inferred information aboutthe quality of the contacts. In another instance, smart drug deliveryapplications may require proper contacts to the body to ensure correctand effective drug delivery. If the contact is improper, drug can poolon the skin due to poor absorption and contact to the skin. Otherapplications, such as electrocardiography or defibrillation, may alsorequire proper contacts to the body. Being able to infer the quality ofthe contacts based on the derived contact impedances can providefeedback to the user regarding the quality of the contacts in suchcontexts as well.

Some efforts to extract contact quality or contact impedance havelimitations, and the schemes for measuring impedances described hereincan improve upon those efforts. In some systems, efforts to extractcontact quality or contact impedance ignore bio-impedance, or assumethat the bio-impedance is zero, close to zero, or very small compared tothe contact impedances. This assumption can be reasonable when theelectrodes are measuring electrical activity of the heart, since in suchsituations, the electrodes are placed close to each other (e.g., on thethorax) and the skin has been prepared to make the body impedance verysmall. The impedances measurement schemes described herein do not makesuch an assumption. Not making this assumption can be beneficial incontexts where the body impedance can be large. For instance, bodyimpedance cannot be ignored when electrodes are placed on other parts ofthe body, far apart from each other, where the bio-impedance can be inthe range of the contact impedances. In another instance, thebio-impedance can be much greater than the contact impedances if theelectrodes have very low impedances. In yet another instance, the lackof skin preparation can also make the contact impedances much largerthan the bio-impedance being measured. For all these reasons, theimpedances measurement schemes described herein can be used in a varietyof situations. For instance, the impedances measurement scheme can beused to, non-invasively, obtain the body's composition, determinethoracic impedance, determine respiration activity in the presence ofmotion, etc.

Method for Measuring Impedances

FIG. 16 is a flow diagram illustrating a method for measuringimpedances, according to some embodiments of the disclosure. Theimpedances include a bio-impedance and four branch impedances. In 1602,circuitry such as mux 112 can form unique signal paths. In 1604, currentmeasurement circuitry 126 can make current measurements of the uniquesignal paths. The unique signal paths setup a system of equations thatenables the impedances to be derived. To ensure the system of equationswould yield the unknown impedances, each unique signal path includes atleast some of the impedances, the unique signal paths include eachimpedance at least once. In 1606, digital circuitry 190 can derive theimpedances based on the current measurements.

Examples

Example 1 is a method for measuring impedances, comprising: formingunique signal paths, wherein each unique signal path includes at leastsome of the impedances, the unique signal paths include each impedanceat least once, and the impedances include a bio-impedance and fourbranch impedances, making current measurements of the unique signalpaths, and deriving the impedances based on the current measurements.

In Example 2, the method of Example 1 can optionally include: deriving avoltage from a signal generator by applying an output of the signalgenerator to a resistor having known resistance value and measuring acurrent through the resistor, and deriving the impedances based furtheron the voltage from the signal generator.

In Example 3, the method of Example 1 or 2 can optionally includeforming signal paths comprising: controlling a configurable network toconnect an output of a signal generator to the unique signal paths andto connect an input of a current measurement circuitry to the uniquesignal paths.

In Example 4, the method of any one of Examples 1-3 can optionallyinclude making the current measurements comprising: applying a signalfrom a signal generator to the unique signal paths, and measuring acurrent through each unique signal paths by a current measurementcircuitry.

In Example 5, the method of any one of Examples 1-4 can optionallyinclude each unique signal path including two branch impedances.

In Example 6, the method of any one of Examples 1-5 can optionallyinclude each one of some of the unique signal paths including thebio-impedance and two branch impedances.

In Example 7, the method of any one of Examples 1-6 can optionallyinclude each unique signal path includes a network of all of theimpedances.

Example 8 is a circuit for measuring impedances, comprising: a signalgenerator to generate a signal at an output of the signal generator,current measurement circuitry to measure a current at an input of thecurrent measurement circuitry, a configurable network to connect theoutput of the signal generator to the input of the current measurementcircuitry through unique signal paths, wherein each unique signal pathincludes at least some of: a bio-impedance and branch impedances, anddigital circuitry to determine the bio-impedance and the branchimpedances based on current measurements of the unique signal paths.

In Example 9, the circuit of Example 8 can optionally include: theconfigurable network being to connect the output of the signal generatorto the input of the current measurement circuitry through at least fiveunique signal paths, and the current measurement circuitry being tomeasure at least five current measurements.

In Example 10, the circuit of Example 8 or 9 can optionally include thedigital circuitry being to determine the bio-impedance and four branchimpedances based on the at least five current measurements of the uniquesignal paths.

In Example 11, the circuit of any one of Examples 8-10 can optionallyinclude the unique signal paths including each one of the bio-impedanceand branch impedances at least once.

In Example 12, the circuit of any one of Examples 8-11 can optionallyinclude each unique signal path including two branch impedances.

In Example 13, the circuit of any one of Examples 8-12 can optionallyinclude each one of some of the unique signal paths including thebio-impedance and two branch impedances.

In Example 14, the circuit of any one of Examples 8-13 can optionallyinclude each unique signal path including a network of all of thebio-impedance and the branch impedances.

In Example 15, the circuit of any one of Examples 8-14 can optionallyinclude the configurable network being to further connect the output ofthe signal generator to the input of the current measurement circuitrythrough a resistor with a known resistance value, and the currentmeasurement circuitry being to further measure a current through theresistor to determine a voltage from the signal generator.

Example 16 is a circuit for measuring impedances, comprising: fourbranches having four electrodes respectively, wherein two of the fourelectrodes are connected to a first end of a bio-impedance, and twoother ones of the four electrodes are connected to a second end of thebio-impedance, circuitry to apply a signal to at least five uniqueimpedance networks and making current measurements of the at least fiveunique impedance networks, wherein each unique impedance network has atleast two of the four branches, digital circuitry to derive thebio-impedance and impedances of the four branches based on the currentmeasurements.

In Example 17, the circuit of Example 16 can optionally include eachunique impedance network including all of the four branches.

In Example 18, the circuit of Example 16 or 17 can optionally includethe at least five unique impedance networks comprising a uniqueimpedance network having one of the four branches connected to a signalgenerator and three other ones of the four branches connected to currentmeasurement circuitry.

In Example 19, the circuit of any one of Examples 16-18 can optionallyinclude the at least five unique impedance networks comprising a secondunique impedance network having two of the four branches connected to asignal generator and two other ones of the four branches connected tocurrent measurement circuitry.

In Example 20, the circuit of any one of Examples 16-19 can optionallyinclude the circuitry being to further connect an output of a signalgenerator to an input of current measurement circuitry through aresistor with known resistance value and to further measure a currentthrough the resistor to determine a measured voltage from the signalgenerator.

In Example 21, the circuit of any one of Examples 16-20 can optionallyinclude the digital circuitry being to determine quality of contactscorresponding to the four electrodes based on the impedances of the fourbranches.

In Example 22, the circuit of any one of Examples 16-21 can optionallyinclude the unique impedance networks including each one of thebio-impedance and impedances of the four branches at least once.

In Example 23, the circuit of any one of Examples 16-22 can optionallyinclude each one of some of the unique impedance networks includes thebio-impedance and impedances of two of the four branches.

In Example 24, the circuit of any one of Examples 16-23 can optionallyinclude each unique impedance networks includes a network of all of thebio-impedance and impedances of the four branches.

Variations and Implementations

The unique signal paths illustrated by the disclosure are not meant tobe limiting. Other topologies, schemes for exciting and measuring thesignal paths can be implemented, and are envisioned by the disclosure.

Moreover, certain embodiments discussed above can be provisioned indigital signal processing technologies for medical imaging, patientmonitoring, medical instrumentation, and home healthcare. Theembodiments herein can also be beneficial to other applicationsrequiring an accurate impedance measurement using at least fourelectrodes.

In the discussions of the embodiments above, various electricalcomponents can readily be replaced, substituted, or otherwise modifiedin order to accommodate particular circuitry needs. Moreover, it shouldbe noted that the use of complementary electronic devices, hardware,software, etc. offer an equally viable option for implementing theteachings of the present disclosure.

Parts of various circuitry for deriving unknown impedances can includeelectronic circuitry to perform the functions described herein. In somecases, one or more parts of the circuitry can be provided by a processorspecially configured for carrying out the functions described herein.For instance, the processor may include one or more application specificcomponents, or may include programmable logic gates which are configuredto carry out the functions describe herein. The circuitry can operate inanalog domain, digital domain, or in a mixed signal domain. In someinstances, the processor may be configured to carrying out the functionsdescribed herein by executing one or more instructions stored on anon-transitory computer medium. In some embodiments, an apparatus caninclude means for performing or implementing one or more of thefunctionalities describe herein.

It is also imperative to note that all of the specifications,dimensions, and relationships outlined herein (e.g., the number ofprocessors, logic operations, etc.) have only been offered for purposesof example and teaching only. Such information may be variedconsiderably without departing from the spirit of the presentdisclosure. The specifications apply only to one non-limiting exampleand, accordingly, they should be construed as such. In the foregoingdescription, example embodiments have been described with reference toparticular processor and/or component arrangements. Variousmodifications and changes may be made to such embodiments withoutdeparting from the scope of the disclosure. The description and drawingsare, accordingly, to be regarded in an illustrative rather than in arestrictive sense.

Note that with the numerous examples provided herein, interaction may bedescribed in terms of two, three, four, or more electrical components.However, this has been done for purposes of clarity and example only. Itshould be appreciated that the system can be consolidated in anysuitable manner. Along similar design alternatives, any of theillustrated components, modules, and elements of the FIGURES may becombined in various possible configurations, all of which are clearlywithin the broad scope of this Specification. In certain cases, it maybe easier to describe one or more of the functionalities of a given setof flows by only referencing a limited number of electrical elements. Itshould be appreciated that the electrical circuits of the FIGURES andits teachings are readily scalable and can accommodate a large number ofcomponents, as well as more complicated/sophisticated arrangements andconfigurations. Accordingly, the examples provided should not limit thescope or inhibit the broad teachings of the electrical circuits aspotentially applied to a myriad of other architectures.

Note that in this Specification, references to various features (e.g.,elements, structures, modules, components, steps, operations,characteristics, etc.) included in “one embodiment”, “exampleembodiment”, “an embodiment”, “another embodiment”, “some embodiments”,“various embodiments”, “other embodiments”, “alternative embodiment”,and the like are intended to mean that any such features are included inone or more embodiments of the present disclosure, but may or may notnecessarily be combined in the same embodiments.

It is also important to note that the functions related to derivingunknown impedances, illustrate only some of the possible functions thatmay be executed by, or within, systems illustrated in the FIGURES. Someof these operations may be deleted or removed where appropriate, orthese operations may be modified or changed considerably withoutdeparting from the scope of the present disclosure. In addition, thetiming of these operations may be altered considerably. The precedingoperational flows have been offered for purposes of example anddiscussion. Substantial flexibility is provided by embodiments describedherein in that any suitable arrangements, chronologies, configurations,and timing mechanisms may be provided without departing from theteachings of the present disclosure.

What is claimed is:
 1. A circuit for measuring impedances, comprising: asignal generator to generate a signal at an output of the signalgenerator, measurement circuitry to make measurements at an input of themeasurement circuitry, a configurable network to couple the output ofthe signal generator to the input of the measurement circuitry throughat least five unique signal paths, wherein each unique signal path formsa network having an impedance of interest and four branch impedances,and digital circuitry to determine the impedance of interest and thefour branch impedances based on at least five measurements of the uniquesignal paths made by the measurement circuitry and a calibrationmeasurement of the signal generator.
 2. The circuit of claim 1, whereinone of the unique signal paths includes two of the branch impedancescoupled in parallel.
 3. The circuit of claim 1, wherein one of theunique signal paths includes two of the branch impedances coupled inparallel with each other, and the two branch impedances coupled inparallel are in series with the impedance of interest.
 4. The circuit ofclaim 1, wherein: a first one and a second one of the four branchimpedances are coupled to a first end of the impedance of interest, athird one and a fourth one of the four branch impedances are coupled toa second end of the impedance of interest, and for one of the uniquesignal paths: the first one of the four branch impedances is coupled tothe output of the signal generator, and the second one, the third one,and the fourth one of the four branch impedances are coupled to theinput of the measurement circuitry.
 5. The circuit of claim 1, wherein:a first one and a second one of the four branch impedances are coupledto a first end of the impedance of interest, a third one and a fourthone of the four branch impedances are coupled to a second end of theimpedance of interest, and for one of the unique signal paths: thesecond one of the four branch impedances is coupled to the output of thesignal generator, and the first one, the third one, and the fourth oneof the four branch impedances are coupled to the input of themeasurement circuitry.
 6. The circuit of claim 1, wherein: a first oneand a second one of the four branch impedances are coupled to a firstend of the impedance of interest, a third one and a fourth one of thefour branch impedances are coupled to a second end of the impedance ofinterest, and for one of the unique signal paths: the third one of thefour branch impedances is coupled to the output of the signal generator,and the first one, the second one, and the fourth one of the four branchimpedances are coupled to the input of the measurement circuitry.
 7. Thecircuit of claim 1, wherein: a first one and a second one of the fourbranch impedances are coupled to a first end of the impedance ofinterest, a third one and a fourth one of the four branch impedances arecoupled to a second end of the impedance of interest, and for one of theunique signal paths: the fourth one of the four branch impedances iscoupled to the output of the signal generator, and the first one, thesecond one, and the third one of the four branch impedances are coupledto the input of the measurement circuitry.
 8. The circuit of claim 1,wherein: a first one and a second one of the four branch impedances arecoupled to a first end of the impedance of interest, a third one and afourth one of the four branch impedances are coupled to a second end ofthe impedance of interest, and for one of the unique signal paths: thefirst one and the second one of the four branch impedances are coupledto the output of the signal generator, and the third one and the fourthone of the four branch impedances are coupled to the input of themeasurement circuitry.
 9. An integrated circuit for measuringimpedances, comprising: first and second pins electrically couplable toa first end of an impedance of interest, third and fourth pinselectrically couplable to a second end of the impedance of interest, asignal generator to generate a signal at an output of the signalgenerator, measurement circuitry to make measurements at an input of themeasurement circuitry, a configurable network to electrically coupleeach one the first, second, third, and fourth pins to either the outputof the signal generator or the input of the measurement circuit to format least five unique closed circuits each having the first, second,third, and fourth pins and the impedance of interest, and digitalcircuitry to determine four branch impedances associated with the first,second, third, and fourth pins, and the impedance of interest based onat least five measurements of the unique closed circuits made by themeasurement circuitry and a calibration measurement of the signalgenerator.
 10. The integrated circuit of claim 9, wherein one of theunique closed circuits includes two of the branch impedances coupled inparallel.
 11. The integrated circuit of claim 9, wherein one of theunique closed circuits includes two of the branch impedances coupled inparallel with each other, and the two branch impedances coupled inparallel are in series with the impedance of interest.
 12. Theintegrated circuit of claim 9, wherein for one of the unique closedcircuits, the first pin is electrically coupled to the output of thesignal generator, and the second pin, the third pin, and the fourth pinare electrically coupled to the input of the measurement circuitry. 13.The integrated circuit of claim 9, wherein for one of the unique closedcircuits, the second pin is electrically coupled to the output of thesignal generator, and the first pin, the third pin, and the fourth pinare electrically coupled to the input of the measurement circuitry. 14.The integrated circuit of claim 9, wherein for one of the unique closedcircuits, the third pin is electrically coupled to the output of thesignal generator, and the first pin, the second pin, and the fourth pinare electrically coupled to the input of the measurement circuitry. 15.The integrated circuit of claim 9, wherein for one of the unique closedcircuits, the fourth pin is electrically coupled to the output of thesignal generator, and the first pin, the second pin, and the third pinare electrically coupled to the input of the measurement circuitry. 16.The integrated circuit of claim 9, wherein for one of the unique closedcircuits, the first pin and the second pin are electrically coupled tothe output of the signal generator, and the third pin, and the fourthpin are electrically coupled to the input of the measurement circuitry.17. A method for measuring impedances, the impedances including animpedance of interest and four branch impedances, comprising: formingfive unique signal paths having all impedances, wherein forming eachunique signal path comprises: (1) coupling a signal generator to asubset of four branches, and (2) coupling remaining branches not in thesubset of four branches to measurement circuitry, making fivemeasurements of the unique signal paths by the measurement circuitry,wherein making each measurement comprises: (1) applying a signal to thesubset of the four branches coupled to the signal generator, and (2)making a measurement of the remaining branches coupled to themeasurement circuitry, and deriving the impedances based on the fivemeasurements.
 18. The method of claim 17, wherein forming the fiveunique signal paths comprises controlling a configurable network tocouple each branch to either the signal generator or the measurementcircuitry, the signal generator being coupled to at least one branch,the measurement circuitry being coupled to at least two branches. 19.The method of claim 17, wherein forming the five unique signal pathsdoes not include grounding any one of the four branches.
 20. The methodof claim 17, wherein deriving the impedances is further based on acalibration measurement of the signal generator.